With the evolution of the High Speed Packet Access (HSPA), in the RAN#50 meeting of the 3rd Generation Partnership Project (3GPP), the close loop transmit diversity (CLTD) was proposed as a work item and the uplink Multiple-Input Multiple-Output (MIMO) was proposed as a study item. For the uplink MIMO, since a plurality of data streams (e.g. two data streams) can be transmitted simultaneously from a user equipment to a base station under good channel quality, the user equipment can obtain a notable gain from the high bit-rate transmission of the uplink. How to flexibly select or determine the number of appropriate data streams (hereinafter referred to as “streams”), e.g. how to determine whether single-stream transmission or multiple-stream transmission is to be conducted in uplink transmission, it involves an uplink rank adaptation technology.
Rank generally represents the number of independent channels for wireless communication between the user equipment and the base station in a multiple-antenna system, while rank adaptation relates to flexibly select from a plurality of ranks a rank for wireless communication between the user equipment and the base station. Taking rank 1 and rank 2 transmission of the uplink as an example, the rank 1 represents that the user equipment transmits data to the base station using a single stream, now the same data is transmitted via different antennas and thereby achieves space diversity, while the rank 2 represents that the user equipment transmits two different streams to the base station and thereby achieves space multiplexing. In addition, the rank with other numerical values may also exist, e.g. rank 4.
In the downlink MIMO of the HSPA, the rank adaptation technology involves that a base station (e.g. a serving Node-B) selects an appropriate rank for downlink MIMO transmission based on the feedback information received from the user equipment, e.g. the user equipment's preferred rank and a Channel Quality Indicator (CQI) as well as a corresponding Pre-Coding Indicator (PCI) for single-stream or multiple-stream transmission. Since enough information including the above information can be obtained at the base station, it would be easy for the base station to determine the rank for the downlink transmission.
Compared to the above case of downlink MIMO rank adaptation, in the case of uplink MIMO rank adaptation, the base station is capable of understanding better about the channel condition, but regarding the base station determining an appropriate rank and a corresponding pre-coding vector, the related information obtained from the user equipment is relatively inadequate and the frequency for obtaining the related information is relatively low. Such related information, for example, may include Uplink Power Headroom (UPH), use equipment buffer status and a transmission grant. For UPH, in the current standard specification, it is reported at a long period (e.g. once 100 ms) or based on event triggering so that the base station will not frequently receive information about UPH. For the use equipment buffer status and the transmission grant, the current user equipment does not report them to the base station. Thus, the base station cannot rapidly obtain sufficient information from the user equipment so as to accurately determine an appropriate rank, and the base station may select an incorrect rank to perform uplink MIMO transmission. Thereby, the uplink MIMO performance is deteriorated and the gain obtained by MIMO is decreased.
In addition, generally speaking, the user equipment should follow the rank determined by the base station for uplink transmission. However, due to the possibility of lacking related information of the user equipment, the rank determined by the base station cannot always be well-suited for the user equipment. Thus, in some cases, the wireless network should allow the user equipment to flexibly change the rank for uplink transmission based on the rank selected by the base station.